Synchronous machines, i.e. synchronous motors and synchronous generators, comprise a rotor and a stator. The rotor and stator interact electromagnetically such that rotation of the rotor is obtained in case the synchronous machine is a motor, and such that current is induced in the stator coil in case the synchronous machine is a generator.
A number of fault conditions may occur in a synchronous machine, both in the stator and in the rotor. Such fault conditions may instantaneously introduce noticeable deviations in behaviour of the synchronous machine, or they may go unnoticed for a longer period of time, which only after months or even years manifests itself in a catastrophic breakdown.
Traditionally, stator current and synchronous machine vibration measurements have been preferred monitoring methods for determining the presence of a fault condition. Stator current measurements must however be performed under the high voltage conditions of the stator, which therefore requires expensive sensor equipment able to withstand such conditions. Furthermore, it has been shown that vibration levels do not reflect synchronous machine conditions in a satisfactory manner.
US20090091289 discloses a fault detection system using field current, i.e. rotor current, and rotor search coil voltage harmonics. The method involves collecting a synchronous machine field current, for example, and predicting a harmonic frequency based on the field current. The predicted harmonic frequency is monitored to determine a status of the stator based on current and voltage. A stator inter-turn fault is detected based on the monitoring of the predicted harmonic frequency, where a ratio of the harmonic frequency to a line frequency is equal to a specific equation. The harmonic frequency is predicted based on a number of fundamental poles of the machine.
US 20090091289 hence provides monitoring and determination of stator inter-turn faults utilising field current measurements. There are however a plurality of possible fault conditions which may arise in a synchronous machine.
U.S. Pat. No. 4,492,999 A discloses a supervisory unit for detecting abnormalities in shaft voltage of rotary electrical machinery and apparatus such as brushless synchronous machines. The supervisory unit includes a first detector for detecting the voltage across at least two points of the rotor shaft, a second detector for detecting the voltage between the rotor shaft and ground, and a third detector for detecting an electrical condition of an AC exciter which constitutes part of the field system of the apparatus. The third detector may detect electrical signals indicative of the field system voltage or the field system current. The supervisory unit includes a voltage comparator and a frequency comparator which receive detection signals from the three detectors to provide a signal indicating abnormality when an abnormal condition occurs, such as contact failure of a grounding brush or a failure in coil insulation.
The paper Wesley Doorsamy et al: “Multiple fault diagnosis on a synchronous 2 pole generator using shaft and flux probe signals”, Industrial Technology (ICIT), 2013 IEEE International Conference ON, IEEE, 25 Feb. 2013, pp. 362-367, discloses a method for diagnosis of multiple incipient faults on a 2-pole synchronous generator is presented. Simulation of the generator on a finite element analysis (FEA) software package is used to predict the effects of these faults. Experimental analysis of the generator under fault conditions is then conducted and confirms the predicted behaviour. The investigation utilises shaft brushes as a non-invasive condition monitoring tool and search coils are used to validate findings from the shaft signal analysis. Results of the investigation indicate definitive relationships between the faults and specific harmonics of the output signals from the condition monitoring tools.